1. Field of the Invention:
The present invention relates to a structural improvement of a surface acoustic wave device capable of reducing undesirable coupling in the device caused by a feed through electromagnetic wave from the input side to the output side.
2. Description of the Prior Art:
Generally, an SAW (surface acoustic wave) element comprises an input piezoelectric transducer and an output piezoelectric transducer, both the transducers having a pair of comb-shaped electrodes which are coupled to each other, and formed on an SAW propagating substrate. The input transducer converts an RF (radio frequency) input electric signal to an SAW propagating on the SAW propagating substrate to the output transducer. And the output transducer converts the SAW propagating from the input transducer to an electric RF signal. The SAW propagating substrate is usually made of such material as a LiNbO.sub.3, quartz or other piezoelectric crystalline material, or a glass having a thin vapor deposited layer of ZnO. The abovementioned SAW element is bonded on a printed circuit substrate, and is packed in a metal package, further being concealed by a cap.
Using a filtering characteristic or a signal delay characteristic, the abovementioned SAW element is used for various electronic device such as a filter device, a delay line, a resonator, etc. Usually, a piezoelectric transducer comprises a pair of comb-shaped electrodes (IDT) which are coupled to each other as the configuration that the hands of opposite electrodes are disposed alternately, and thereby the transducer can be made sufficiently small, comparable to a wavelength (10.sup.-5 times a wavelength of the radio wave) of the SAW. Such IDT are formed by photo-etching.
A conventional SAW device comprises the abovementioned SAW element as shown in FIG. 1, wherein a SAW element 3 is bonded for example by an adhesive on a printed circuit substrate 1 having a grounding conductor layer 2 on a surface thereof.
An input piezoelectric transducer 4 and an output piezoelectric transducer 5 are disposed on the substrate 1, and also an input wiring conductor 6 and an output wiring conductor 7 are formed on the surface of the substrate 1. The input transducer 4 and the input conductor 6 are connected with each other by connecting means such as wire-bonding by using such a wire as Al wire, Au wire, etc. And the output transducer 5 and the output conductor 7 are connected with each other similarly.
Another conventional SAW device is shown in FIG. 2. In FIG. 2, a SAW 3' is diebonded on a conductive surface region of a can package 8. An input piezoelectric transducer 4' and a pair of input lead wires 10,10' are connected with each other by wire-bonding. And also an output piezoelectric transducer 5' and a pair of output lead wires 9,9' are connected with each other by wire-bonding. However, concerning an SAW element having transducers of abovementioned structure and disposition, a feed through electromagnetic wave from the input transducer to the output transducer increases as operating frequency becomes high. The feed through electromagnetic wave has a bad influence on various characteristics of the SAW element.
Let us analyze an operating characteristic of the SAW element using an equivalent circuit of a device including the SAW element mounted in a can package.
In FIG. 3, Zs is an electrical equivalent impedance of the input transducer, and Zs' is an electrical equivalent impedance of the output transducer. Es is an electromotive force induced in the output transducer. Rwa, Rwa', Rwb, and Rwb' are equivalent resistances of the wiring for the SAW element including the bonding wires. Lwa, Lwb, Lwa' and Lwb' are equivalent inductances of the wirings for the SAW element including the bonding wires. Ca, Cb, Ca', Cb', Cc, Cc', Cd and Cd' are equivalent stray capacities between the transducers and the grounding electrodes. Rei, Re and Reo are equivalent resistances of the grounding electrodes of the package. Ro is a grounding resistance between input and output both of which are outside of the package.
E is an input signal voltage considering a matching circuit outside of the package. Z is an impedance of a load connected to the output side of the device considering the matching circuit. In the equivalent circuit, the input transducer and the output transducer have a common grounding conductor, therefore an input signal current flows through the resistance Re, Reo and Ro. Accordingly, some voltage drop is induced across the resistance Reo responding to the input signal current. That is, an electromotive force caused by the feed through electromagnetic wave responding to the input signal is generated. Hence, when the resistance Re is infinitely large and none of stray capacities is in parallel with the resistance Re, the voltage drop in the resistance Reo will not occur. And when the resistance Re is infinitely large, and by means of a capacitive coupling caused by the stray capacities Cc, Cc' between the input transducer and the output grounding electrode and/or the stray capacities Cd, Cd' between the output transducer and the input grounding electrode, a considerable voltage drop in the resistance Reo occurs in the conventional structure. Hence it is desirable that the disposing between the input transducer and the output grounding as well as disposing between the output transducer and the input grounding electrode should be apart from each other.